1. Field of the Invention
The present invention generally relates to a scanning electron microscope (SEM) device, and, more particularly, to a critical dimension scanning electron microscope (CD-SEM) device and a method of measuring a pattern size obtained by the CD-SEM device.
2. Description of the Related Art
For today""s ultra-fine semiconductor devices, a design rule of 0.1 xcexcm or smaller is being used. To produce such ultra-fine semiconductor devices, it is necessary to form a pattern having a line width of 0.1 xcexcm or smaller.
By a conventional method of measuring a pattern width by a CD-SEM, an electron scanning region (or a field of view) is scanned with electrons, and secondary electrons released from the sample to be measured are subjected to brightness conversion by the scintillator. The converted amount of the secondary electrons is displayed. In a CD-SEM, the brightness level is used for obtaining image data and line-width data.
FIG. 1 is a flowchart showing a conventional method of changing the magnification and measuring the pattern size by a CD-SEM, and FIGS. 2A to 2C illustrate the method shown in FIG. 1.
As shown in FIG. 1, a magnification of 50K (K is 103) is selected in step S1. Here, the scanning range of an electron beam is indicated by a region 1 shown in FIG. 2A. This region 1 includes a pitch line pattern 2 formed on a device chip (a sample). In step S2, the scanning range is irradiated with the electron beam. In step S3, secondary electrons released from the sample are sent to a scintillator.
In step S4, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region 1 obtained by the CD-SEM device at the 50K magnification is outputted to a CRT. In step S5, the line width (the measured value) of the pitch line pattern 2 measured by the CD-SEM device at the 50K magnification is outputted to the CRT.
If desired information cannot be obtained from the image outputted to the CRT in step S4, the magnification is increased. For instance, a magnification of 100K is selected in step S6. As shown in FIGS. 2A and 2B, the electron beam scanning range is indicated by a region 3. In step S7, the scanning range constituted by the region 3 is irradiated with an electron beam. In step S8, secondary electrons released from the pitch line pattern 2 are captured by the scintillator.
In step S9, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region 3 obtained by the CD-SEM device at the 100K magnification is outputted to a CRT. In step S10, the line width (the measured value) of the pitch line pattern 2 measured by the CD-SEM device at the 100K magnification is outputted to the CRT.
If desired information is not obtained from the image outputted to the CRT in step S9, the magnification is further increased. For instance, a magnification of 150K is selected in step S11. As shown in FIGS. 2B and 2C, the electron beam scanning range is indicated by a region 5. In step S12, the scanning range constituted by the region 5 is irradiated with an electron beam. In step S13, secondary electrons released from the pitch line pattern 2 are supplied to the scintillator. In step S14, based on a signal generated by brightness conversion performed by the scintillator, an image that reproduces the region 5 obtained by the CD-SEM device at the 150K magnification is outputted to the CRT. In step S15, the line width (the measured value) of the pitch line pattern 2 measured by the CD-SEM device at the 150K magnification is outputted, and the operation then comes to an end.
In the above operation, the timing of the scintillator capturing the secondary electrons forms uniform intervals. For instance, the capturing intervals may be 10xe2x88x927 (sec). With the electron irradiation amount per unit area in the region 1 at the 50K magnification shown in FIG. 2A being normalized to 1.0, the electron irradiation amount per unit area in the region 3 at the 100K magnification shown in FIG. 2B is 4.0, and the electron irradiation amount per unit area in the region 5 at the 150K magnification shown in FIG. 2C is 9.0.
As described above, as semiconductor chips have rapidly become smaller, there is an increasing need to produce a CD-SEM having a high magnification so as to measure the line widths of an ultra-fine pattern. With a higher magnification, a resolution per pixel becomes higher, and a high-precision SEM image and pattern measurement can be carried out.
However, the above method causes problems that hinder high-precision measurement of pattern widths. More specifically, to increase a measuring magnification, the area of an electron beam irradiation region or a field of view (FOV) is reduced. If the FOV is small while the magnification is high, the quantity of electron irradiation per unit area on the surface of the sample increases, and a large amount of electrons is applied to a small area. As a result, the influence from charges and contamination becomes greater at a high magnification. For instance, there will be problems that the contrast varies to make the object (a measured pattern) look darker, and that the line widths in a reproduced image are different from the original.
The influence from charging can be eliminated by reducing the current density at a high modification. With a reduced current density, however, there will be other problems, such as focus deviation of a scanning electron beam and a decrease in S/N ratio of an obtained brightness signal. As for the contamination, a material such as amorphous carbon in the chamber is polymerized with incident grains, and accumulates on the sample. To reduce the adverse influence from the contamination, the vacuum degree in the chamber is increased, and a technique, such as a cold trap technique, for collecting amorphous carbon, has been employed, but no sufficient results have been reported to this date.
As described so far, with the conventional techniques, it is difficult to obtain high-precision SEM images and to perform accurate line width measurement, because of the strong influence from the charging and contamination at a high magnification.
Japanese Laid-Open Patent Application No. 10-213427 discloses a method of measuring a size of a circuit pattern for a short period of time by reducing damage and charges applied on the circuit pattern on the wafer. However, this method is utilized for scanning a smallest possible area for measuring the edges of a circuit pattern. As the magnification becomes higher, the quantity of electron irradiation per unit area becomes larger. As a result, the problem of deterioration in measurement accuracy remains. Also, the entire area is subjected to electron irradiation so as to determine a scanning range, resulting in contamination (or carbonization) on the scanned area.
A general object of the present invention is to provide scanning electron microscopes in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a scanning electron microscope that can constantly obtain a high-precision SEM image and a pattern width measurement value, without damaging an object to be measured even at a high magnification, and a method of measuring a pattern size using the scanning electron microscope.
The above objects of the present invention are achieved by a method of controlling a scanning electron microscope, the method comprising the steps of: irradiating an object with an electron beam; and detecting electrons generated from the object due to the irradiation, at a frequency depending on a magnification for observing the object.
By this method, a desired image can be obtained regardless of the detection magnification.
The above objects of the present invention are also achieved by a method of controlling a scanning electron microscope, the method comprising the steps of:
irradiating a surface of a sample object with an electron beam; and
detecting electrons released from the surface of the sample object due to the irradiation,
wherein:
a first scanning range in a first direction of two different directions on the surface of the sample object is selected in accordance with a detection magnification on the surface of the sample object, while a second scanning range in a second direction of the two different directions is fixed; and
the electron detection is performed at intervals T=(FOV1/FOV2)xc3x97t1, with the first scanning range being FOV1, the second scanning range being FOV2, and an initial value of the intervals being t1.
By this method, a desired image can be obtained regardless of the detection magnification.
The above objects of the present invention are also achieved by a scanning electron microscope including:
an irradiating unit that irradiates an object with an electron beam; and
a detecting unit that detects electrons released from the object due to the irradiation, at a frequency depending on a magnification at which the object is observed.
The above objects of the present invention are also achieved by a scanning electron microscope that irradiates a surface of a sample object with an electron beam so as to detect electrons released from the surface of the sample object due to the irradiation, said microscope comprising:
a scanning unit that determines a first scanning rage in a first direction of two different directions on the surface of the sample object in accordance with a detection magnification for the surface of the sample object, while maintaining a second scanning range in a second direction of the two different directions constant; and
a detection timing determining unit that determines intervals T for detecting electrons by (FOV1/FOV2)xc3x97t1, the first scanning range being FOV1, the second scanning range being FOV2, and an initial value of detection intervals being t1.
With the above scanning electron microscope, the magnification can be increased without increasing the quantity of electron beam per unit area on an object to be measured. Accordingly, the object to be measured can be prevented from being affected by adverse influence from charging and contamination, and all the problems that might be caused by the adverse influence can be avoided. Furthermore, since the object to be measured is protected from damage, a high-precision image and a pattern size measured value can be constantly obtained.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.